Vaccination is the administration of antigenic material (a vaccine) to stimulate an individual's immune system to develop adaptive immunity to a pathogen. Vaccines can prevent or ameliorate morbidity from infection. When a sufficiently large percentage of a population has been vaccinated, this results in herd immunity.
The effectiveness of vaccination has been widely studied and verified; for example, the influenza vaccine, the HPV vaccine, and the chicken pox vaccine.Vaccination is the most effective method of preventing infectious diseases; widespread immunity due to vaccination is largely responsible for the worldwide eradication of smallpox and the restriction of diseases such as polio, measles, and tetanus from much of the world. The World Health Organization (WHO) reports that licensed vaccines are currently available to prevent or contribute to the prevention and control of twenty-five preventable infections.

Although now at the forefront of molecular technologies, vaccination has a history in the West going back at least 200 years (vacca - 'cow' refers to Edward Jenner's use of cowpox against smallpox). The basic idea is unchanged: to introduce one or more foreign antigens in the hope of stimulating a protective immune response to destroy or disable the pathogen. Often this approach seeks to mimic the natural immunity to infection which develops to diseases of poverty, sometimes after only a single episode.
Whatever the mechanism, a vaccine has three components:
Antigen(s) - these are parts of the pathogen introduced to stimulate a specific, protective immune response
Adjuvant - this is given to boost the immune response, for example by presenting the antigen in a certain way
Platform - this is the mechanism (e.g. viral vector) by which the vaccine is introduced and (if applicable) is replicated within the host
Modern vaccines are of several broad types according to how the antigen is introduced.
Attenuated vaccines contain either a live but weakened (non-pathogenic) form of the whole pathogen, or a pathogen which has been killed.
Subunit vaccines and virus-like particles contain proteins from the pathogen, produced either by purification or - more commonly today - in an engineered bacterium such as E. coli.
Recombinant Deoxyribonucleic Acid (DNA) vaccines contain some or all of the gene(s) for expression of the antigen(s) within a suitable molecule.
Although some of the most successful vaccines have been live attenuated forms (e.g. measles vaccine, oral polio vaccine), most modern vaccines are based on recombinant DNA technologies in some shape or form. They often seek to express several key antigens simultaneously within the host.